World Environment Day was on 5th June 2013 and marks one hundred years of South Africa's battle against so-called 'Alien Invasives'. These plants strangle out South Africa’s natural flora are are a constant target of teams of weed controllers.

Armed with carefully selected agents – insects that eat only the target plant – these dedicated scientists have fought to save the country’s biodiversity from alien invasive plants that have been brought across oceans from as far as Asia, South America, and the US.

“In most cases these plants were brought into the country for aesthetic reasons, or as curiosities. The problem is that there is nothing in South Africa’s natural environment that targets these plants, so they multiply uncontrolled,” says entomologist Professor Marcus Byrne from Wits University.

Alien invasive species pose a serious threat to South Africa’s biodiversity, and by implication, its economy. Farmers lose crops and livestock, and in a country that already has a water scarcity, 7% of water is disappearing to weeds like black wattle (Acacia mearnsii) and satansbos (Solanum elaeagnifolium).

Weed control involves going back to the native country to find an agent that limits the growth of the plant in its natural environment. There are very specific requirements for an insect (or fungus) to be selected as an agent.

“The agent cannot target anything other than the alien invasive species, otherwise you risk doing more harm than good. What’s incredible about weed control in South Africa is that – over a 100 year period – we haven’t made a single mistake,” says Byrne.

Mistakes can be costly. In 1935, Australia imported about 3 000 cane toads in the hope that they would control the destructive cane beetle population. They turned out to be failures at controlling the beetles, but remarkably successful at reproducing, now numbering well into the millions, and eating everything including pet food left outside homes.

But weed controllers in South Africa haven’t put a foot wrong. In fact, they’ve saved the country millions of rands, as recognised by the fact that government is starting to make more funds available via the Working for Water programme.

“In the case of the golden wattle, for every rand spent on weed control, the country has saved R4 333,” says Byrne.

The chain-fruit cholla tree, which is indigenous to Arizona, Texas and parts of Mexico, forms dense stands of spiny, branched, tree-sized succulent plants with easily-detachable stem segments. Birds, reptiles and small mammals are frequently impaled on the long, barbed spines and suffer a cruel death. Game, livestock and pets become so covered in spiny segments that they eventually die.

South African Dr Helmuth Zimmermann, world expert on the bio-control of cacti, took his wife to see a chain-fruit cholla tree, and they witnessed the suffering of a small antelope that accidentally ran into it.

“Zimmermann made a promise to this wife that he would help eradicate this cactus. He did this by finding the highly damaging biotype of cochineal that is now eradicating the cactus,” says Hildegard Klein from the Plant Protection Research Institute.